Spray the lawn now. Let it die and begin to decompose. Give it a few weeks to a month. Then mow it very short. Don'tmow before spraying it unless it is very tall. The more grass you spray, the more chemical will get to the roots. You do not need to allow more weeds to germinate and spray a second time. You should deal with weeds next spring. But if you were to start right now, perhaps you could coax some more weed seeds to germinate so you can spray them later this month. That doesn't mean you won't still have weeds coming up before winter, though.

After you have mowed short, rake up the lawn and set the clippings aside. You can use some of them to mulch your seed, or you can distribute them somewhere else.

As for the seed, I suggest you visit a good local, independently owned garden center and get their recommendations. They often sell their own mixes/blends that are optimized for your area.

A slit seeder is not necessary for all grass types so I would ask about that when you buy your grass seed. The best time to plant isn't the same every year, but the 15th is probably about right.

+1 on dealing with weeds in the spring.

As for topdressing, I think it can be a good idea but it's a huge amount of effort and is often expensive. That said, it depends a lot on what you're using and the kind of soil you have already. I read a study that found adding 1/4" of mushroom compost every year was beneficial.

Have you considered aeration? It can help with certain types of soil (and hurt with others). What about organic fertilizers such as alfalfa or soy husks?

Research and find good seed mix for my area and buy it. Any pointers for MA? How much seed for 1500 sq ft?

KBG should be seeded at 3lbs./k. Fescue or Rye should be seeded at 6-8 lbs./k

1. Aug 22 - spray lawn with glyphosate (forgot brand name, its made by bonide)
2. water the lawn to let some weed seeds grow
3. Sep. 7 - spray again with glyphosate

Other than a possible issue with the timeing--no problem.
By all means do a second kill after watering a week after the first kill.

4. Sep 15 - seed the lawn with slit seeder. (Should I remove the dead plant material from the lawn?)

It is adviseable to reveal bare soil for best seed to soil contact. A fixed blade slit seeder can be used for this, but raking is also effective.

I can confidently recommend slit seeding for improved KBG germination. Spread the KBG seed and run the fixed blade slit seeder set at a depth of 1/4" over the lawn to bury the seed. 2passes at 45 degree angles or more passes if you have the stamina.

I do not advise slit seeding for fescue or rye. Although I have always seen improved germination with KBG, I have not seen such results with fescue and rye. Although It may be beneficial to use a slit seeder to break the soil surface for seed to soil contact, a slit seeder should not be run over fescue seed as I believe it actually damages the seed.

5. water the lawn regularly

Heep the seed/soil damp, do not drown the seed. If there is puddling you are overwatering.

6. Enjoy my awesome lawn come spring
questions:
Should I mow the lawn low before I apply glyphosate?

No, the greater the leaf surface available for the glyphosate, the better the kill.
How about this timetable?

You may be a bit late, especially with this mild Summer. You want to work backwards. You want the new grass to need at least 3 mowings so it is established enough for Winter survival. KBG can take 4 weeks to germinate and fescues two weeks.

Should I start earlier or later?

I'd start ASAP with the kill.

soil is tested and extension dept did not recommend amendments. Any advantages to top dressing after overseeding?

Many people swear that topdressing with no more than 1/4" of peat moss will improve success. I agree.

Anything I am missing?
Apply a starter fertilizer at seeding. Scotts staarter with mesotrione may help prevent unwanted weeds. And Winterize--see my post "The Book on KBG."

Good luck to you on your project.

Buy your chemicals for grass or bug kill at Arbico Organics. The chemicals are the same, except original product names are used, plus you can get a stronger solution. Round Up is usually like 12% active, you can get it up to 95% active from AO. I've saved thousands buying from these knowledgeable folks, talk to them by calling their number at the site. Bing or google it and good luck to you, just follow the instructions. JV

Thanks for the advice.

Planning to spray this weekend. (too much rain when I had time and dry when I was busy). Planning to seed in first week of Sept based on the advice here.

MulchMama - so grass dead from roundup is OK as mulch? Also don't know any local seed seller, so probably will end up on internet.
Is usual rake enough to rake out the dead leaves or should I get the steel dethacher?

Thiking of using KBG + TTTF mix. (here is the thread)

smkyle - I aerated the lawn last year, so not planning to do it again this time.

thanks grass1950. I need to figure out if I can/should use slit seeder with the mix I am planning (mostly TTTF). I thought it would be better because it dumps the seed under the soil for better soil contact. I had not thought of using starter fert. Will do that with the seeding.

To add to the advice here. I would highly recommend that you pick up Scotts Starter Fertilizer with Weed Preventer that contains Mesotrione. This is the active ingredient in Tenacity and will help prevent any weed seeds from germinating. For a 1500 sq ft lawn, a $30 bag should allow you 2 full applications, 5 weeks apart to maximize the effectiveness of the Tenacity. (Tenacity is effective for roughly 6 weeks at the concentration recommended by Scotts).

Of course, you could buy Tenacity yourself and spray with a backpack/tank sprayer, but dosing to the right concentration can be tricky and for such a small lawn, I don't think it would be worth the hassle.